Joint initiation in bedded elastic rocks

Two general types of sedimentary structures serve as initiation points (IP) for cross-fold joints in siltstone beds of the Ithaca Formation, Appalachi an Plateau. New York: planar bed boundaries and geometrically complex featu res such as bedforms, trace fossils, and soft sediment deformation structur es. The relative abundance of each IP type varies with the: stratigraphic p osition of the jointed body. When they are mechanically isolated by thicker shales, siltstone bodies have a higher percentage of IP associated with co mplex features. This association supports the hypothesis that sedimentary s tructures serve to concentrate stress, thus becoming preferred sires for pr imary joint initiation. Secondary joint initiation occurs at planar bed bou ndaries through interaction with pre-existing joints in adjacent siltstone beds that are mechanically coupled to form composite joints. The propagatio n path of joints passing through planar bed boundaries of composite joints can be traced backward to primary IP at complex sedimentary structures. Mod eling suggests that when beds are mechanically coupled, the modest joint-ti p stress concentration across a share layer (e.g.. < 3 x for a l-cm thick b ed) is equivalent to the stress concentration in the vicinity of a bedform. Consequently. in mechanically coupled siltstone beds, sedimentary structur es such as bedforms compete with preexisting joint tips in adjacent beds to initiate new joint segments. (C) 2001 Elsevier Science Ltd. All rights res erved.